Three-Photon Fluorescence Microscopy Shows Deeper Cellular Structures

A horizontal frame from 3-D reconstructed three-photon microscopy images in a mouse brain. The red is neurons, the blue is blood vessels, and the dark holes are non-fluorescent neurons.

Two-photon excitation microscopy is a technique developed at Cornell that’s been used to image living tissue at cellular level to a depth of about a millimeter. Its big problem is the infrared light that’s needed will scatter quickly, creating noise. By having more photons involved in absorption by the tissue, higher resolution at greater depth can be achieved.

Following up on the work of their colleagues, a team at Cornell developed three-photon fluorescence microscopy. Using the new technique, they were able to visualize in 3D the subcortical region of a live mouse brain. The vasculature within the hippocampus becomes clearly visible as well as neurons that were labelled with red fluorescent protein dye.

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Using a mouse model, the researchers have proved the principle of three-photon microscopy operating at a wavelength of 1,700 nanometers. This, in combination with the new laser specifically created for three-photon excitation, allows the researchers to perform high-resolution imaging of neurons at unprecedented depths within a mouse brain.

Pushing these depth limits is important for basic science and eventually could prove useful clinically, Xu said. Depression and diseases like Parkinson’s and Alzheimer’s are associated with changes deep inside the brain, and finding the cures could be helped by subcortical neural imaging — that is, below the gray matter of the brain and into the white matter and beyond, if the brain is visualized as stacked layers.

“Brain mapping could be the so-called grand challenge within the next decade,” Xu said. “With MRI, we can see the whole brain but not with the resolution we have demonstrated. The optical resolution is about 100 to 1,000 times higher and allows us to clearly visualize individual neurons.”

Press release: Three-photon microscopy improves biological imaging

Study abstract in Nature Photonics: In vivo three-photon microscopy of subcortical structures within an intact mouse brain